12AP4 Rebuilding

Some time Dave
Johnson contacted Clinton Electronics in Rockford, Illinois about the
possibility of rebuilding pre-1945 picture tubes. We sent an email to Randy
Schroedl, describing the project:

Jeff Aulik suggested that I write to you concerning my
attempts to get several pre-1945 CRTs rebuilt. As you probably
know, tubes made before the war used Pyrex or some other high
temperature glass, and, as a result, can't be rebuilt using
standard methods.

There have been a couple of successful rebuilds in the past.
They were done by welding several pieces of glass tubing, each
with a lower melting temperature, to the bulb. The last piece of
tubing could be welded to a modern stem. In this approach a
1960's electrostatic focus gun was used.

I would like to attempt a different approach. I believe I can
get the original guns rebuilt by John Bishop (formerly of
Global). Then, I'd have stems made of high temperature glass.
All that would be needed is to get a piece of tubing of the same
glass and the rebuild could be done like a normal tube. Of
course higher temperatures would be involved in the process.

There are quite a few 9AP4 and 12AP4s out there that need
rebuilding. I have a database of the surviving sets using these
tubes, and I could contact the owners to see how many need
tubes, or want spares. The one good 12AP4 that came on the
market recently (not new, but with reasonable emission) sold for
over $1500, so it would be worth quite a bit to a collector to
get a new tube.

In addition, I have a number of other pre-1945 tubes that need
to be rebuilt, and I'm sure there are other collectors with
other tubes.

Successful rebuilding would also guarantee that the 400 or so
surviving pre-1945 sets will be operational far into the future.
Without a suppy of tubes, they will all be dark within a few
years.

Please let me know if you have any interest in persuing this.
Thanks for your help.

Randy responded

Thank you for your inquiry, we will review your request
and advise within the next week. There is always a concern with
rebuilding CRTs with potential implosions as a result of bruises
or scratches on glass that has been in the field. I am not sure
how this issue is currently handled but we would not be in a
position to take responsibility for any shrinkage as a result of
our attempt to rework the CRT. We are also interested in any
input you may have to determine to what extent the CRTs must be
reworked once we receive them (i.e. from a simple re-sealing to
a complete rework). Your input will help us to determine the
project feasibility

Dave Johnson agreed to provide a dud 12AP4 for them
to work with. After a few weeks, and several emails about sources
for rebuilt guns and Pyrex stems, Randy reported on their progress:

I just wanted to give you an update on our progress. We
were able to remove the gun and getter assembly from the dud
without problems. We believe we have located the proper
neck tubing to reneck the CRT and we are currently awaiting
tubing delivery. I faxed a proposed stem drawing to ETI for
review and as long as they can come up with something close
with the right glass we should be able to use it. We have
run some initial screening test to be in position re-screen
and aluminize the CRT for 6kv operation. A review of the
e-gun shows we may have the parts in-house to build an
equivalent design once we have the proper stem. We will
know more once the tubing arrives for sealing tests.

Today we received the first samples of a Pyrex-compatible
stem for the 12AP4. We will start evaluating them and
hopefully, the stem is the last high hurdle in this
project.

Then, almost a year passed without any progress. In April of 2005 Randy
again reported on their progress:

The re-build progress is going slower than we anticipated. We
have all the major hurdles covered but just need to get more time
on our production sealing equipment. We can now seal the Pyrex
glass but must work on reducing the residual strain. We are
hoping to get a little time on the production equipment later this
week and should have more time available on the equipment next
week. We have the process and materials defined for reworking the
glass, re-screening for the low anode operation, new modern gun
design with a Pyrex stem, exhaust cycle defined with auto stem tip
off, and just need to complete the sealing procedure. We still
remain optimistic that we can run the older tubes on our equipment
and any one that gets one of our re-built tubes will have a tube
that looks brand new capable of many hours of use.

So, it appears that we are close to having a place to
get pre-1945 tubes rebuilt. There are limitations to the type of tubes
they can do. Only magnetic deflection tubes with necks about 35mm in
diameter are possible, but this includes the 9AP4, 12AP4, and a number
of British 7, 9 and 12 inch tubes. Later that month, Randy wrote:

As you know from Randy's last e-mail, one significant
problem we are working on is sealing the electron gun into the
tube. We have made a number of good seals but they all develop a
crack within an hour or so. There is too great a difference in the
coefficient of expansion of the Pyrex stem and the Pyrex neck
tubing; there are different types of Pyrex which have different
coefficient of expansion rates.

In July, 2005 Randy informed us that the project had been
put on hold, because the company didn't want continue to fund the
experiment with no guarantee of success. Glass was located in Germany
that Randy felt might solve the problem. The Early Television
Foundation offered to pay the $800 cost for the glass to keep the
project going. Randy wrote in July:

You are correct that the
project is currently on hold. We have put a lot of engineering
time and material cost into the project with limited success.
Management current has no additional material expenditures
budgeted for the project. Your proposal to fund the special
neck tubing to proceed with the development was well received by
our management. We have been approved to allocate additional
engineering time to the project if you fund the tubing. We feel
the tubing is the key to solving our next hurdle but like all
projects "Murphy" is always around the corner. We are finding
very little information on the material properties for the
prewar Pyrex which has made the progress a lot more difficult to
develop the process. We know the tubing we have located will be
a closer match to the available Pyrex glass stems but we are
still guessing on the actual Pyrex glass that was used to make
the pre-1945 bulbs. Once we solve the technical issues we feel
the actual price to re-build the old pre-1945 tubes and convert
them to an aluminized screen will run in the $700 to $750 price
range. If you feel the market is willing to pay this price for the
pre-1945 Pyrex version then the tubing is worth pursuing.

I replied that I was confident that collectors would
pay $700-750 for a rebuilt pre-1945 tube. As a result, the glass has
been ordered and the project is underway again.

In February, 2006 Randy reported
that the German glass had successfully been welded to the 12AP4 glass.
An attempt will be made within the next few weeks to rebuild a 12AP4.

After several attempts, Clinton was unable to keep the stem from
cracking as it cooled, and determined that a custom stem would be
required. So far, Clinton has been unable to find a stem at a reasonable
cost.

We have some glass tubing that was used in a previous
successful rebuild of a 9AP4. This tubing is of an intermediate melting
temperature, and can be welded to both the Pyrex shell and the soft
glass stem. Clinton is considering trying this approach.

Clinton has sold its rebuilding operation to Video Display Corp., and is
no longer interested in pursuing this project. However, a French
company, RACS, has recently been successful in rebuilding a Pyrex tube.